This proposal describes research to be undertaken through a cooperative agreement between the Yale Arbovirus Research Unit, USA, the National Institute of Hygiene, Venezuela, and the University of La Plata, Argentina. The P.I. of the parent grant contributed to the discovery of a new arenavirus, Guanarito, the etiologic agent of Venezuelan hemorrhagic fever (VHF), during the course of this project. this FIRCA application aims to extend the evolutionary studies on RNA viruses circulating in Venezuela. The specific aims of this proposal are: (1) to determine the evolutionary relationship of guanarito virus to 2 other arenaviruses circulating in South America, 2) to produce rapid diagnostic tests for the identification of guanarito virus in clinical samples, through the use of PCR and hybridization probe techniques, and (3) to implement these new protocols in the Venezuelan laboratory whose responsibility it is to distinguish this virus from others circulating in the country, such as dengue (clinical symptoms similar to VHF) and Venezuelan equine encephalitis (apparently same rodent reservoir as VHF). It is anticipated that this collaboration will lead to expanded research cooperation between United States, Argentinian and Venezuelan scientists on the epidemiology, treatment and control of this new disease.